PROJECT DESCRIPTION: Autosomal dominant hypercholesterolemia (ADH) is a dominantly inherited disorder present from birth that causes marked elevation in plasma low-density lipoprotein cholesterol (LDL-C) and premature coronary heart disease (CHD). Despite the clear benefit of LDL-C lowering with regards to CHD prevention, less than 10% of cases with ADH have been diagnosed, and only 5% of ADH patients are adequately treated. ADH is genetically heterogeneous and has been associated with variants in at least three genes including LDL receptor (LDLR), apolipoprotein B-100 (APOB), and proprotein convertase subtilisin-like kexin type 9 (PCSK9). Systematic screenings to identify ADH patients with mutations in these genes has been undertaken in several countries through nationwide screening programs and lipid clinic networks. These studies have found that approximately 5%-53% of patients with a clinical phenotype of ADH do not have mutations in any of the three known loci. Most of these studies were performed in ethnically and racially homogenous populations from Europe or Japan. Thus, there is paucity of data about the molecular basis of ADH in multi-ethnic cohorts, especially African Americans. Our preliminary data in a multi-ethnic cohort of 93 unrelated ADH patients revealed a strikingly high percentage (66%) of patients without any variants in LDLR, APOB and PCSK9 (unexplained ADH). Interestingly, the frequency of unexplained ADH was much higher among African Americans (77%) than among non-Hispanic whites (57%) or Hispanics (53%). Therefore, we hypothesize that mutations in novel genes may underlie a large proportion of ADH in multi-ethnic cohorts, especially of African descent. The overall aim of this project is identify new genetic defects responsible for hypercholesterolemia in patients who have an undetermined etiology of ADH. To address this aim, we propose to study 400 patients with ADH. Patients will be ascertained from specialty lipid clinics, and their entire families will be recruited. DNA will be screened with standard sequencing techniques for mutations in the exons and consensus splice sites of LDLR, APOB and PCSK9 or by linkage analysis using informative markers at these loci. Deletions and duplications of LDLR exons will be detected with multiplex ligation-dependent probe amplification technique. Based on preliminary data, we expect to establish a cohort of 240 patients with ADH of unknown etiology. To rapidly identify new ADH genes in this cohort of unexplained ADH patients, we will utilize a hybrid method that combines next generation sequencing with positional cloning. Elucidation of new genes with major effects on lipid metabolism will shed light on the molecular processes that lead to development of high plasma levels of LDL-C and premature CHD. The studies outlined in this project will provide the basis for the development of new therapeutic options for the prevention of this potentially life-threatening chronic disease.